In a TOF-MS, ions accelerated by an electric field are injected into a flight space where no electric field or magnetic field is present. The ions are separated by their mass-to-charge ratios according to the total flight time until they reach and are detected by a detector. Since the difference of the lengths of flight time of two ions having different mass-to-charge ratios is larger as the flight path is longer, it is preferable to design the flight path as long as possible in order to enhance the resolution of the mass-to-charge ratio (or mass resolution) of a TOF-MS. In many cases, however, it is difficult to incorporate a long straight path in a TOF-MS due to the limited overall size, so that various measures have been taken to effectively lengthen the flight length.
In the Japanese Unexamined Patent Publication No. H11-195398, an elliptical orbit is formed using plural toroidal type sector-formed electric fields, and the ions are guided to fly repeatedly in the elliptical orbit a number of times, whereby the effective flight length is elongated. In this TOF-MS, as the number of turns the ions fly in the orbit increases, the flight distance is larger and the length of flight time is accordingly longer, so that the mass resolution becomes better by increasing the number of turns.
When, as described above, ions repeatedly fly in a loop orbit, ions having smaller mass-to-charge ratios will gain higher speeds. Therefore, ions having a smaller mass-to-charge ratio may lap other ions having larger mass-to-charge ratios while they are orbiting. If the detector simultaneously detects a group of ions mixed with different number of times, it is impossible to determine the mass-to-charge ratios of the ions without knowing the number of turns of each ion.
The Japanese Unexamined Patent Publication Nos. 2000-243345 and 2003-86129 disclose conventional mass spectrometers constructed to avoid the previously described problem, in which gradually shifting the ions every time they lap the orbit forms a flat spiral flight path. More specifically, the TOF-MS disclosed in the Japanese Unexamined Patent Publication No. 2000-243345 includes a flight space having a polygonal orbit created by a circular arrangement of electric field segments obtained by dividing a cylindrical electric field, and the angle of injecting ions into the electric field segment located at the entrance is appropriately determined so that the ions gradually shift in the axial direction of the cylindrical electric field while they orbit in the cylindrical electric field. The TOF-MS disclosed in the Japanese Unexamined Patent Publication No. 2003-86129, also having a polygonal flight space, generates a deflecting electric field between a pair of adjacent electric field segments for gradually shifting the flying ions in the axial direction of the cylindrical electric field. When the flight path of an ion is spiral, the terminal position of the ion in the axial direction of the cylindrical electric field gradually changes with the number of turns of the ion. Therefore, it is possible to detect such ions that have flown in the orbit a predetermined number of turns by extracting the ions at an appropriate position and introducing them into the detector.
In the above-described TOF-MSs, the number of turns of the ion introduced into the detector is basically determined by the construction of the electrodes for generating the deflecting electric field and the position at which the ions are extracted. This means that the mass resolution of the aforementioned conventional TOF-MSs is fixedly determined by their construction because the mass resolution depends on the number of turns. Therefore, in some cases, ions having different but very close mass-to-charge ratios cannot be separated from each other.